The subject matter disclosed herein relates to electric braking apparatuses and, more particularly, to electric braking apparatuses with damping end-stops.
Electric braking for aerospace applications requires the use of electro-mechanical actuators (EMAs) to apply axial loads on brake stacks. System requirements mandate that these actuators be able to fully retract on command. The fully retracted position of an EMA is defined by an “end-stop.”
EMAs often make use of ballscrews to convert torque to the desired axial force necessary to operate the brake stacks. When EMAs use ballscrews, the ballscrews are often designed with an end-stop feature that is used to positively stop the EMA in its fully retracted position. The end-stop feature allows the EMA to have “re-homing” position control as well as a known service position for brake stack replacement.
Among the requirements for the end-stop feature is the requirement that the end-stop feature be capable of stopping a full-force retract command in the event of a software failure. Typically, end-stop features must be designed with substantial tab wall thicknesses as the forces generated due to a full-force retract command include motor and system inertia loading that can both create substantial impact forces. These impact forces must be absorbed by the end-stop features and mechanical or material failures of end-stop features due to overloading are common.
In some cases, the impact forces can be sudden and cause the ballscrew balls to accelerate and wedge into track grooves whereby the ballscrew is prevented from turning. Also, the impact forces can cause excess forces to be exerted on the EMA gearing and motor, which could lead to an EMA jam. If an end-stop feature fails, the ballscrew has the potential to disassemble itself and fall out of the actuator. This could lead to foreign object damage (FOD), which is a failure mode that is unacceptable in the Aerospace industry. Thus, end-stop features are often provided with excessive design margins that lead to larger and heavier actuator packages.